For manufacturing a cast product having a columnar hole such as a bore in an engine cylinder block, it is customary to insert a core into a cavity of a casting die, cast a molten metal into the cavity, remove the core after the molten metal is solidified, and release the casting from the die, so that the casting with a columnar hole defined therein is produced. In order to remove the core smoothly, the core needs to have a certain draft angle for removal. However, since the bore has to be of a gradient-free cylindrical shape, it is necessary to cut the casting depending on the draft angle. If the draft angle is large or the columnar hole is deep, then the amount of material machined off the casting is large when the casting is cut, the time required to cut the casting is long, and many chips are produced, resulting in a reduction in the rate of utilization of the material. Generally, castings tend to contain more blowholes in deeper regions from the surface. Therefore, if a large amount of material is machined off the casting, then many blowholes are liable to appear in the cut surface of the casting.
The amount of material machined off the casting should preferably be small, and the draft angle of the core should desirably be zero. To meet these demands, there has been proposed a die apparatus having a core which comprises an inner member and an outer member whose tapered surfaces are slidably supported on opposite side surfaces of the inner member (see, for example, Patent No. 3406266 (Japan)).
The proposed die apparatus allows the core to be removed smoothly while being kept out of interference with the bottom wall of a space in the product. Only a side core member of the core is movable radially inwardly for removal of the core. Therefore, the side core member does not require a draft angle on its outer circumferential surface. However, if the columnar hole is deep, then though the side core member is releasable, another core member may not be releasable and needs to have a draft angle.
For using a casting as an engine cylinder block, the casting should preferably be processed by a hard coating process in view of sliding movement of pistons. However, if the casting has a draft angle, then after the inner surface of the casting is cut, blowholes appear on the cut inner surface and may possibly prevent a hard coating process from being properly performed on the inner surface. When a casting is heated, the surface of the casting may possibly be unduly deformed due to blowholes that are present on or near the surface of the casting.